RDMA/mlx5: Add attr for max number page list length for PI operation

PI offload (protection information) is a feature that each RDMA provider
can implement differently. Thus, introduce new device attribute to define
the maximal length of the page list for PI fast registration operation. For
example, mlx5 driver uses a single internal MR to map both data and
protection SGL's, so it's equal to max_fast_reg_page_list_len / 2.

Signed-off-by: Max Gurtovoy <maxg@mellanox.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>